Lubricating oil compositions used to lubricate internal combustion engines contain a base oil of lubricating viscosity, or a mixture of such oils, and additives used to improve the performance characteristics of the oil. For example, additives are used to improve detergency, to reduce engine wear, to provide stability against heat and oxidation, to reduce oil consumption, to inhibit corrosion, to act as a dispersant, and to reduce friction loss. Some additives provide multiple benefits, such as a dispersant/viscosity modifier. Other additives, while improving one characteristic of the lubricating oil, have an adverse effect on other characteristics. Thus, to provide a lubricating oil having optimal overall performance, it is necessary to characterize and understand all the effects of the various additives available, and carefully balance the additive content of the lubricant. Moreover, each of the additives is a separate component of the formulated lubricating oil and adds cost. Thus, it would be beneficial to have a multi-functional additive that controls more than one performance characteristic of the lubricating oil.
To provide improved wear performance, conventional lubricants are formulated with an anti-wear additive. Metal hydrocarbyl dithiophosphates, particularly zinc dialkyldithiophosphates (ZDDP), are the primary anti-wear additive used in lubricating oils for internal combustion engines. ZDDP provides excellent wear protection at a comparatively low cost and also functions as an antioxidant. However, there is some evidence that phosphorus in lubricants can shorten the effective life of automotive emission catalysts. Accordingly, industry has limited the amount of phosphorus that lubricants can contain. The previous industry category (ILSAC GF-3) mandated a lubricant phosphorus limit of 1000 ppm. The current category of service fill oils in the United States (ILSAC GF-4) mandates even more stringent limits, a maximum phosphorus content of no more than 800 ppm. Future standards may require even more stringent limits, such as a maximum phosphorus content of no more than 500 ppm.
To meet these emerging requirements, it would therefore be advantageous to provide lubricating oils, particularly lubricating oils formulated with base oils having relatively high viscosity indices and low volatilities for improved fuel economy, that also provide excellent low temperature valve train wear performance and anti-oxidant properties with reduced amounts of phosphorus-containing anti-wear additives.
U.S. Pat. Nos. 5,346,635 and 5,439,605 describe lubricating oils completely free of phosphorus-containing anti-wear additives containing a complex blend of ashless friction reducers, ashless anti-wear/extreme pressure additives, antioxidants, metal detergents and polymeric viscosity modifiers and flow improvers, which compositions purportedly provide acceptable properties. These compositions may also contain a molybdenum-containing additive as a friction modifier.
U.S. Pat. No. 5,523,008 (the '008 patent), which is incorporated herein by reference in its entirety, describes polyolefins containing nitrogen-containing and/or oxygen-containing monomers for use as dispersant viscosity index improvers. The grafted polyolefin contains at least about 13 moles of N-vinylimidazole (similarly known as 1-vinylimidazole), or other ethylenically-unsaturated nitrogen-containing and/or oxygen-containing monomers, per mole of a grafted polyolefin backbone. Also described is a lubricating oil comprising a lubricant base oil and a grafted polyolefin as described above as well as a method of making a dispersant-viscosity index improver. N-vinylimidazole or other ethylenically unsaturated nitrogen-containing and/or oxygen-containing monomers and a graftable polyolefin are reacted with sufficient initiator to graft at least about 13 moles of the monomer to each mole of the polyolefin.
The inventors of the present invention have now discovered a novel multi-function graft polymer useful as an additive for lubricating oil compositions. The multiple function graft polymer operates as a dispersant viscosity index improver, as well as an anti-wear additive without having an adverse effect on anti-oxidant properties.